Portable vs Booth Audiometer: Do Calibration Rules Differ?

A clinic audiometer bolted to a desk next to a sound booth and a portable unit that rides in a mobile test van every week look like different instruments, and safety managers often assume they carry different calibration rules. Under OSHA's noise standard they do not. 29 CFR 1910.95(h)(5) sets one schedule for every audiometer used in a hearing conservation program: a functional check each day of use, an acoustic calibration at least annually, and an exhaustive calibration at least every two years. What does differ is how fast a portable unit can drift, how often its test environment changes, and how easily its calibration gets broken by a swapped earphone. This post compares the two and covers the extra habits a portable unit needs to stay inside the rule.

Are OSHA calibration intervals different for portable audiometers?

No. The calibration paragraph in 1910.95(h)(5) does not mention portability, mobile vans, or booths. It requires a daily functional check "by testing a person with known, stable hearing thresholds" and listening for distorted or unwanted sounds, with deviations of 10 dB or more requiring an acoustic calibration. It requires an acoustic check at least annually under Appendix E, with deviations of 15 dB or more requiring an exhaustive calibration, and an exhaustive calibration at least every two years under ANSI S3.6. Those intervals apply the same way to a laptop-driven portable unit and a fixed clinic audiometer. The only mobile van exception in 1910.95 is about timing for the baseline audiogram: (g)(5)(ii) allows up to one year after first exposure instead of six months when mobile vans are used. It gives no relief on calibration. A portable audiometer that skipped its annual acoustic check is out of compliance in exactly the same way a booth unit would be.

Is any federal rule stricter for audiometers in mobile vans?

One is, and it is outside OSHA. The Federal Railroad Administration's hearing conservation rule for railroad operating employees, 49 CFR 227.111(d)(3), requires an exhaustive calibration under ANSI S3.6-2004 "at least once every two years on audiometers not used in mobile test vans" and "at least annually on audiometers used in mobile test vans." The FRA rule also requires the annual acoustic check to hold the sound pressure output within 3 dB between 500 and 5000 Hz and 5 dB at 6000 Hz and higher. That rule binds railroads, not general industry, but it reflects a practical point: an audiometer that rides in a vehicle takes vibration, temperature swings, and connector wear that a desk unit never sees. Employers outside the railroad industry are still held to the OSHA two-year interval, though some choose the annual exhaustive schedule for vans voluntarily. A railroad contractor whose employees are tested in a van should confirm the provider calibrates to the FRA schedule.

Why does a portable audiometer need more frequent ambient noise checks?

Because its test room moves. Appendix D of 1910.95 caps background levels in any audiometric test room at 40 dB at 500 and 1000 Hz, 47 dB at 2000 Hz, 57 dB at 4000 Hz, and 62 dB at 8000 Hz, measured in octave bands. A fixed booth in a quiet clinic rarely changes. A van parked beside a compressor house on Monday and next to a highway on Tuesday faces a new noise floor at every stop. The NIOSH-published hearing measurement chapter suggests measuring a fixed room annually but taking ambient levels in mobile facilities daily or whenever the facility is relocated, whichever is more frequent. Those readings matter for validity: 1910.95(g)(7)(iii) requires the employer to give the reviewing professional the test room background levels, and (m)(2)(ii)(F) requires accurate records of them. Foster's post on mobile testing without a booth covers the ambient side in more detail.

What breaks the calibration on a portable audiometer?

The most common failure is not the electronics. It is the earphones. Appendix E measures the sound pressure level an audiometer delivers through a specific earphone placed on a coupler, so the calibration belongs to that audiometer and earphone pair. Swapping in a spare headset from another unit, or replacing a cracked cord with a generic one, means the output the employee hears is no longer the output that was measured. Portable units also share the risks that the NIOSH chapter lists for any audiometer, including calibration drift and interfering signals from the test equipment, with the added wear of daily packing, cable flexing, and cold starts in a vehicle. A shift that shows up across several employees tested on the same portable unit on the same day points to the instrument, not to their ears. Foster's guide to signs an audiometer is drifting walks through how that pattern shows up.

How Foster calibrates and tracks portable units

Foster's audiometer calibration service calibrates each audiometer with the earphones it actually uses, so the certificate describes the setup the employees will hear. For Foster's own mobile units, the current acoustic calibration certificate travels with the van, the daily functional check is logged against each test date, and an octave-band ambient reading is taken at each parking location before the first audiogram. That record set answers the questions in 1910.95(m): the date of the last acoustic or exhaustive calibration and the background levels in the test room. Customers who own a portable unit can ship it in or schedule it with a site visit through the calibration page, or ask for a quote on on-site testing that brings a calibrated van to the plant.

Frequently asked questions

Does a portable audiometer need its own sound booth to be calibrated?

No. Acoustic calibration under Appendix E uses a sound level meter, an octave-band filter set, and a coupler; it measures the audiometer and earphone output, not the room. The room is checked separately against Appendix D.

If a van audiometer gets dropped, can it keep testing?

Run the functional check before testing anyone. If any frequency is off by 10 dB or more, 1910.95(h)(5)(i) requires an acoustic calibration before the unit is used for program audiograms.

Can we swap earphones between two calibrated audiometers?

Treat it as breaking both calibrations. Each certificate covers the earphones that were on the coupler during calibration.

Do insert earphones change the calibration rules?

The intervals stay the same. The reference levels differ by transducer type, so the calibration must be done with the inserts the unit actually uses.

Sources

• 29 CFR 1910.95, Occupational noise exposure, including Appendices D and E (eCFR)
• 49 CFR 227.111, Audiometric test requirements (FRA railroad rule)
• Franks, J.R., Hearing Measurement, NIOSH/CDC Stacks